1. Field of the Invention
This invention relates generally to rolling mills, and is concerned in particular with an improvement in single strand block type finishing mills of the type employed in the twist-free rolling of rods, bars and other like products.
2. Description of the Prior Art
An example of a well-known single strand block type rolling mill is disclosed in U.S. Pat. No. 4,537,055, the disclosure of which is herein incorporated by reference in its entirety. In mills of this type, as herein further depicted schematically in FIGS. 1-3, successive roll stands ST.sub.1 -ST.sub.10 are alternately arranged along opposite sides of the mill pass line P. The roll pairs R.sub.1 -R.sub.10 of the successive roll stands are oppositely inclined and appropriately grooved to roll the product in an oval-round sequence and in a twist-free manner.
The output shaft 10 of a mill drive motor 12 drives the center gear 14 of a speed increaser 16. Gear 14 in turn drives a pair of side gears 18, 20 carried on line shafts 22,24 extending in parallel relationship to the mill pass line P. Segments of the line shafts extend through and are journalled for rotation in the roll stands, with their adjacent protruding ends being externally coupled to each other by couplings 26. Because of the staggered relationship of the roll stands, roll stand ST.sub.9 is spaced from the speed increaser 16 by a gap which is bridged by a Cardan shaft segment 24a.
With reference in particular to FIGS. 2 and 3, it will be seen that each line shaft segment located within a roll stand carries a drive bevel gear 28 which meshes with a driven bevel gear 30 carried on one of two parallel intermediate drive shafts 32. The intermediate drive shafts carry intermeshed spur gears 34. The work rolls R are removably mounted in cantilever fashion on the ends of parallel roll shafts 36. Each roll shaft carries a pinion gear 38 which meshes with one of the spur gears 34. The spur and pinion gears 34, 38 are thus arranged in what is commonly referred to as a "four gear cluster".
Although not shown, it will be understood that adjustment means are internally provided at each roll stand for adjusting the parting between the work rolls. Such adjustment means typically shift the roll shafts 36 and their pinion gears 38 symmetrically in opposite directions in relation to the mill pass line, while allowing the intermediate drive shafts 32 and their intermeshed spur gears 34 to remain undisturbed. Guides (also not shown) are provided between the successive work roll pairs to guide the product along the mill pass line. Conventionally, the spacing "C" between successive work roll pairs (commonly referred to as the "stand center" distance) will be on the order of 600-800 mm.
In a typical modern high speed rod rolling operation, a 16-24 mm round will be delivered to stand ST.sub.1 from an upstream intermediate mill (not shown) at a speed of about 8-18 m/sec, and will exit from the last stand ST.sub.10 as a finished 5.5 mm round at a speed of around 100 m/sec. The ratios of the successive bevel gear sets 28,30 and four gear clusters 34,38 are selected to accommodate the rapidly accelerating product and to insure that the product is under a slight tension as it progresses through the mill.
Conventionally, the cross section of the product exiting from the finishing block will be within tolerances which are acceptable for some but not all purposes. For example, a properly rolled 5.5 mm round will have a tolerance at or slightly below the limit of .+-.0.15 mm as specified by ASTM-A29. Such products may be used "as is" for many applications, including for example welding mesh, chicken wire, etc. For other uses, however, such as for example valve steels, much tighter tolerances on the order of 1/4 ASTM are required. Such products are commonly referred to as "precision rounds". In the past, this level of precision has been achieved either by subjecting the bars to a separate machining operation after the rolling operation has been completed, or by continuously rolling the bars through additional separately driven "sizing stands".
The separate machining operations, commonly referred to as "peeling", add significantly to the cost of the finished products. Although continued rolling through sizing stands is less costly, the relatively light reductions taken in each sizing pass at a location downstream from the finishing block appear to encourage unacceptable levels of grain growth, which in extreme cases require remedial action in the form of separate and costly heat treatments.